During braking of a vehicle, a load on the vehicle shifts in the forward direction. This load shift increases a load on wheels at the front in the forward direction (front wheels) and decreases a load on wheels at the back in the forward direction (rear wheels). For this reason, in a case where the same brake force is applied to the front and rear wheels, the rear wheels given the smaller wheel load tend to lock up before the front wheels given the larger wheel load (hereinafter, also referred to as “preceding lock-up of the rear wheels”).
If the front wheels lock up first, problematic unstableness does not occur in the vehicle's behaviors because the rear wheels at the back in the inertial direction have a grip on the road surface. On the other hand, if the rear wheels lock up first as described above, the vehicle may become unable to exert a sufficient grip in the vehicle width direction. Since the front wheels at the front in the inertial direction have a solid grip on the road surface, there occurs a problem that the vehicle spins about the front wheels, causing unstableness in behaviors thereof.
Moreover, since the front wheels are given a larger wheel load, the application of a larger brake force to the front wheels does not cause the lock up thereof; however, because the rear wheels tend to lock up before the front wheels as described above, the brake force of the front wheels cannot be used to a full extent despite the amount of free brake force left unused. This leads to a lack of brake force of the vehicle as a whole and causes a problem that the driver cannot obtain an expected vehicle deceleration through his/her brake operation.
In view of the above problems, the brake forces are generally distributed to the front and rear wheels in such a way that the brake force may be smaller at the rear wheels, which tend to lock up first, than at the front wheels. In this way, the tendency of the preceding lock-up of the rear wheels may be reduced, solving the unstableness in the vehicle's behaviors attributable to the preceding lock-up of the rear wheels. In addition to this, reducing the tendency of the preceding lock-up of the rear wheels makes it possible to fully use the brake force of the front wheels given a larger wheel load. Hence, the driver can obtain an expected vehicle deceleration through his/her brake operation.
However, the following problems occur if the above brake-force distribution control is applied as it is to a case of moving the vehicle backward.
During braking while moving the vehicle backward, a load on the vehicle shifts in the backward direction. In contrast to the earlier case, this load shift increases the wheel load on wheels at the front in the backward direction (the rear wheels described above) and decreases the wheel load on wheels at the back in the backward direction (the front wheels described above). This creates an unused, free portion in the brake force of the wheels at the front in the backward direction (rear wheels), and makes the wheels at the back in the backward direction (front wheels) tend to lock up first.
If, despite this tendency, the control using the same brake-force distribution as the forward movement is performed, so that the brake force is set smaller for the wheels at the front in the backward direction (rear wheels) than for the wheels at the back in the backward direction (front wheels). The brake forces are distributed to the front and rear wheels in an opposite way to how the weight shifts in the vehicle. Accordingly, the brake force of the wheels at the back in the backward direction (front wheels) becomes far larger than that without the above-described brake-force distribution control. This makes the problem of the preceding lock-up tendency (and therefore unstable behaviors) more severe. In addition, since the brake force of the wheels at the front in the backward direction (rear wheels) cannot be fully used, the problem that a vehicle deceleration the driver expects cannot be obtained becomes more severe as well.
In view of such problems occurring during braking while moving the vehicle backward, there has been proposed a related art such as one described in PL 1, for example. According to this related art, when braking is performed while the vehicle is moving backward, a motor regenerative brake force is applied to the wheels at the front in the backward direction (rear wheels) correspondingly to the increase in the load on these wheels, so that the above problems occurring during the braking in the middle of backward movement may be alleviated.